Dense accumulations of synaptic-like microvesicles in 'dark' pinealocytes of the gerbil pineal gland.

In an electron-microscopical study the occurrence and ultrastructural features of electron-dense 'dark' variants of pinealocytes were evaluated in the gerbil pineal gland. A few 'dark' pinealocytes, which tended to form small clusters of contiguous cells, could consistently be detected in pineals fixed and embedded by various procedures. Apart from the different degree of electron density, the only conspicuous difference between 'dark' and electron-lucent 'light' pinealocytes concerned their compartment of synaptic-like microvesicles. Thus, both variants of pinealocytes contained abundant clear microvesicles of variable size which accumulated in dilated process terminals. However, the vesicles within the process endings of 'dark' pinealocytes showed an unusually dense arrangement throughout the cytoplasm. As was demonstrated by immunogold staining, the accumulations of vesicles in the 'dark' terminals contained synaptophysin, a major synaptic vesicle-associated protein. This protein is present in small clear vesicles with putative secretory functions in a wide variety of neuroendocrine cells and has previously been shown to be a common constituent of microvesicles in mammalian pinealocytes. Since gerbil pinealocytes displayed distinct gradations of electron density, their ultrastructural heterogeneity may be the expression of different states of secretory activity of one pinealocyte cell type. On the other hand, differences in the content of synaptic-like microvesicles in the process terminals of 'light' and 'dark' cells could also indicate a principal functional heterogeneity of the microvesicular compartment among pinealocytes, pointing to the existence of different types of pinealocytes.